- People in my contact list (or whitelist) automatically get through
- Everyone else is forwarded to a real live human who screens my calls
(The reason why I would pay is I'd rather just have a well-paid human handle this instead of some kind of AI or heuristic.)
The human could be paired with some data, like what phone network the call came from, and additional data about how likely caller ID is spoofed. If you want to get fancy, you could do some kind of machine learning where you tell the person, "this call is xx% likely to be spam."
To keep things simple, I shouldn't be able to set many preferences. (This allows the human to react the same way every time.) But, things like choosing to allow or block "good" robocalls, like automated calls from the power company, are probably useful.
You could try to make a profit by actually asking to get on the do-not-call list, tracking it, and then aggressively suing violators. When I'm interrupted on my cell phone, I just don't have the time to track this kind of thing.
And, one more thing: If you want to get into AI, see if you can figure out how to keep the spammers on the phone as long as possible. This increases their costs dramatically.